Method of making glass-ceramic articles

ABSTRACT

This invention relates to the production of glass-ceramic articles having compositions within the Li2O-Al2O3-SiO2 field which are modified through the addition of ZnO in combination with a second metal oxide (RmOn) selected from the group consisting of SrO, BaO, Y2O3, La2O3, and mixtures thereof. When nucleated with TiO2, beta-spodumene solid solution crystals are formed as the predominant crystal phase with minor amounts of such secondary crystal phases as gahnite, rutile, or anatase, and complex crystals associated with the second metal oxides also being present. The resultant products exhibit low coefficients of thermal expansion and excellent creep resistance up to temperatures of 1,000*C. and higher. This desirable combination of properties can frequently be achieved where the heat treatment to produce the glass-ceramic body is of relatively short duration.

[ METHOD OF MAKING GLASS-CERAMIC ARTICLES l [75] Inventor; Richard F. Reade, Corning, NY.

[73] Assignee: Corning Glass Works, Corning,

[22] Filed: May 26, 1972 [21] Appl. No.: 257,324

[52] US. Cl. 65/33, lO6/39.7 51 Int. Cl C03b 29/00 [58] Field of Search 65/33; 106/39.7

[56] References Cited UNlTED STATES PATENTS 3,582,385 6/1971 1 Duke era1.;...=...-.-...: 65/33 3,732,116 5/1973 Reade 65/33 X Primary Examiner-Frank W. Miga Attorney, Agent, or Firm-Clinton S. Janes, Jr'.

[ May 28, 1974 [57] ABSTRACT This invention relates to the production of glass- 'ceramicarticles having compositions within the Li O-Al O -SiO field which are modified through the addition of ZnO in combination with a second metal oxide (R,,,O,,) selectedfrom the group consisting of SrO, BaO, Y O M1 0 and mixtures thereof. When nucleated with TiO- beta-spodumene solid solution crystals are formed as the predominant crystal phase with minor amounts of such secondary crystal phases as gahnite, rutile, or anatase, and complex crystals associated with the second metal oxides also being present. The resultant products exhibit low coefficients of thermal expansion and excellent creep resistance up to temperatures of 1,000C. and higher. This desirable combination of properties can frequently be achieved where the heat treatment to produce the glass-ceramic body is of relatiyely short duration.

' 3 Claims, No Drawings '1 A glass-ceramic article contemplates the controlled crystallization in situ of a glass article through the heat treatment thereof. The steps of-rnanufacture of a glassceramic article are three. First, a glass-forming batch including, commonly, a nucleating agent is melted. Second, that melt'is simultaneously cooled and an article of glass shaped therefrom. Third, the glass article is exposed to a heat treating procedure wherein nuclei are initially developedtherein which provide sites for the growth of crystals thereon as the heat treating is continued.

Since a glass-ceramicarticle is the result of crystal development effected by means of substantially simultaneous growth on countless nuclei distributed throughout the precursor j glass article, the structure thereof is free of voids, nonporous, and consists of relatively uniformly-sized, fine-grained crystals homogeneously dispersed in a glassy matrix, the crystals com-- prising the predominant proportion of the article. Accordingly, glass-ceramic articles are normally characterized as being greater than 50% crystalline and, often, are actually greaterthan 90% crystalline. Inasmuch as a glass-ceramic article is commonly very highly crystalline, the physical and -.chemical properties demonstrated thereby will more closely approximate those of the crystals than those of the residual glassy matrix. Furthermore, the very highcrystallinity leads to the residual glassy matrix having a composition which is much different chemically from that'of the parent glass article since the-components making up the crystals will have been precipitated therefrom.

The crystal phases grown within an individual glassceramic article is a functionofboth the original glass composition and the schedule of heat treatment to which the glass article is subjected. A rather. complete discussion of the theoretical concepts and the practical considerations involved in the production of glassceramic articles can be found in U.S. Pat. No. 2,920,971 towhich reference is hereby :made for such information.

U.S. Pat. No. 3,582,385'describes the production of glass-ceramic bodies exhibiting coefficients 'of thermal expansion (25 -900C.) less than l5 X l"/C. and good dimensional stability upon long time exposures to temperatures up to 800C. Such bodies were'produced through the crystallization in situ of glass bodies having compositions within the Li 'O-BaO-Al O -SiO field which. when nucleatedxwith TiO and. optionally ZrO would be converted to glassrceramic bodies containing beta-spodumene solid solution and celsian as the primary crystal phases. :Thus, the base glass consisted essentially, by weight onthe oxide'basis, of 3.5-5% Li O, 2.5-5-7r BaO. -21% ."AliO 65-75% SiO and 35-89? of the nucleatingagents.

That patent also pointedout that glass-ceramic articles displaying low coetflcients of thermal expansion due to the presence of betaspodumene as the principal crystal phase were well-known to the art. However, dimensional stability upon 1 thermal cycling had been lacking in such bodies. until-thediscovery of the disclosed LEO-BaO-AlgOrSiO :compositions. Finally, that patent emphasized theneedtodelete those metal oxides from the basecompositionwhich would tend to form secondary'crystalphases whose solid solubility -2 with beta-spodumene varies as a function of temperature. The three such metal oxides mentioned were MgO, ZnO. and B 0 Likewise, the presence of Na O and K 0 was eschewed since these promoted the development of residual glass which could also reduce high temperature stability. Nevertheless, where only Li O, Al O and SiO: comprised the base glass, cracking of the body during the crystallization heat treatment frequently occurred. Hence, some fluidity was required to be maintained in the glass to allow stress release during the initial crystallization of the glass to beta-quartz solid solution where there is a great exothermic reaction accompanied with a large change in density. The beta-quartz solid solution is converted to betaspodumene solid solution as the heat treatment is continued. Furthermore, the simple Li O-Al O -SiO glass compositions tended to exhibit secondary grain growth in the absence of a second crystal phase which would inhibit such growth.

Therefore, the primary teaching of that patent was two-fold. First, the BaO remainedin the glassy phase during the initial crystallization which reduced the tendencyto cracking. Second, as the heat treatment was continued, the BaO entered into the crystallization process bycausing the growth of celsian which inhibited grain growth without causing thermal instability. This latter phenomenon decreased the amount of residual glassy matrix in the final product which led to a consequent improvement in thermal stability.

Whereas the glass-ceramic articles disclosed in that patent do, indeed, illustrate low coefficients of thermal expansion and excellent dimensional stability at temperatures up to 800C., there has been the need for compositions which would exhibit good dimensional stability and creep resistance but at temperatures ap- I proaching l,000C. U.S. application Ser. No. 82,844, now U.S. Pat. No. 3,732,116 filed Oct. 21, 1970 by the present inventor, describes one group of such products.

That application disclosed the fact that glass-ceramic articles consisting essentially solely of Li O, A1 0 and SiO which, when nucleated with TiO and, optionally, ZrO to thereafter be crystallized in situ to yield betaspodumene solid solution as substantially the only crystal phase present, exhibited very low coefficients of thermal expansions and high viscosities, at elevated temperatures. "Nevertheless, such articles were extremely'difficult to crystallize-in situ without cracking occurring when the cross section of the articles wasof any substantial thickness. This cracking was caused by sharp stress differentials being set up in the body as the initial crystal phase'formed-at lower temperatures, viz., beta-eucryptite solid solution, crystallized and was then transformed to beta-.spodumene solid solution as the heat treating temperature was raised. The highly-' siliceous, residual glassy phase was so viscous at the temperatures of crystallization that flow therein adequate to permit stress release was inhibited such that cracking could result.

Therefore, to provide means for decreasing the viscosity of the glassy phase during the initial crystallization step such as to reduce the hazard of cracking, a minor amount of at least one of the following metal oxides was included in the glass composition: SrO, V 0 Isa- 0 and Ta O Furthermore, the inclusion of these metal oxides in the compositions led to the growth of very minor amounts of crystal phases wherein the metal hibit a markedly improved behavior with respect to cracking during the initial crystallization sequence, considerable care was still required to insure the complete absence of crackingp Furthermore, problems in forming the original glass bodies from the molten batches are encountered.

Furthermore, in general, the compositions'disclosed in US. Pat, No. 3,582,385 and application Ser. No. 82,844, supra, are difficult tomelt and form into intricateshapes and require long heat treating schedules to fully develop all the crystal phases and assure that the residual glassy matrix is at a minimum.

Therefore, the primary'objectives of this invention are four:

1. to'provide glass compositions which exhibit a desirable liquidus viscosity that will facilitate the forming of intricate shapes;

2. to provide glass compositions which'display excellent'resist'ance to cracking during'the crystallization heat treatment; 1

3, to provide glass-ceramic bodies which demonstrate low coefficients of thermal expansion and good resistance to creep at temperatures up to 1,000C.; and

4. toprovide glass compositions capable of being described in 115. application Ser. No. 82,844, supra, but it appears that they can also retard the frequently excessively rapid crystallization of the initiallydeveloped beta-quartz or beta-eucryptite' solid solution phase, thereby promoting even more effective stress release. i

The extent of the Zl'lO-fOf-LlgO molar substitutionmust be carefully controlled to insure that: 1 undesir v able crystal phases such as cristobalite will not be developed; (2) any minor phases'associated with the secondary. metal oxides can crystallize without interference; and (3) gahnite will crystallize during the heat treatment. These requirements severely limit the extent of the ZnO-for-Li O substitution. Hence, the molar ratio ZnO:Li O must be held between about 1:4 1:9 with a ratio of 1:6.5 beinggreatly preferred. Furthermore, inasmuch as the secondary metal oxides tend to soften or flux the molten glass, their substitution for Li O is to be avoided so as to permit the full utilization of the liquidus temperature reducing effect and gahnite development afforded by the ZnO-forLi O substitution.

. sional stability at high temperatures, the A1 0 content crystallized to glass-ceramic bodies exhibiting those desired properties through relatively short heat treating schedules; 1

The inclusion of ZnO in Li O-Al O -SiO glasses results in the lowering of the liquidus temperature of such compositions without introducing asubstantial softening effect, i.e.', a significant decrease in the viscosity of the melt-at any given temperature. This, in essence, means that the viscosity of themelt at the liquidus has been effectively increased. This phenomenon enables intricate shapes. such as multi-bore tubing for regenerative heat exchanger units or emissions control substrate bodies. to be formed at a more favorable, lower temperature-higher viscosity environment without hazarding premature, uncontrolled devitrification.

The crux of the instant invention lies in the discovery that, within a narrow range of Li-,0-A1,0,-si0, compositions containing TiO, as the nucleating agent, a strictly defined mole-for-rnole substitution of ZnO for Li O will lead to the development of ,aminor quantity of gahnite crystallization (Zn0-A-1 O in addition to the principal beta-spodumene solid solution. This gahnite phase exhibits excellent thermal stability, its presence being observed to persist without dissolution up to temperatures as high as"1,200C. Furthermore, the inclusion of ZnO-in't'hej compositionsalsolowers the liquidus temperature of the melt without softening the glass to any substantial extent.

' Also, thecombination of ZnO with a second metal oxide (R,,,O,,) selected from the group consisting of SrO, BaO, Y O La O and mixtures thereof, appears to impart tothe glass body extraordinary resistance to cracking during the crystallization heat'treatment. The

ZnO and the secondary metal oxide seemingly act in.

concert to not only causea fluxing action, such as was is compelled to be adjustedto compensate for the addition of the secondary metal oxides. Hence, the A1 0 content must be such that the molar ratio Al O :(Li O ZnO nR,,,O,,) must be maintained above 0.95 and, preferably, higher than unity. Where that ratio assumes a value less than 0.95, the resultant glass-ceramic bodies frequently exhibit inferior'microstructure, phase development, and/or physical properties.

Therefore, glass-ceramic articles illustrating coefficients of thermal expansion (25-900C.') of less than about 10 X 1O /C andexcellent resistance to creep at temperatures up to 1,000C. can be produced through the heat treatment of glass bodies consisting essentially, by weight on'the oxide basis, of about 62-73% SiO 15-23% A1 0 3-5% Li O, 1-3% ZnO, 3-6% TiO and l-5% R,,,O,,, wherein R,,,O,, consists of SrO, BaO, Y O La O and mixtures thereof, and wherein the molar ratio ZnO:Li O varies between about 1:4 1:9 and the molar ratio Al O :(Li- O ZnO nR,,,O,,) is greater than 0.95. The heat treatment schedule contemplates nucleation in the 750- 900C.

range and crystallization in the 900l25()C. range. Specific time-temperature schedules for nucleation and crystallization are essentially limitless in number. In general, however, periods for nucleation will range between about l/4-6 hours and for crystallization will range between about 1-12 hours. Longer'nucleation and crystallization times, of course, can be employed with no deleterious effect on the final product and can, in some instances, actually improve the high temperaturestability thereof. However, commercial practice ing the required internal microstructure and physical properties consistent with the intended application.

Theabove-delineated amounts of Li O, ZnO, A1 0 SiO R,,,O,,, and nucleating agent are demanded to secure glasses having the desired forming properties and which will resist cracking during crystallization in situ as well as to obtain glass-ceramic articles exhibiting coefficients of. thermal expansion below about 10 X 1O /C. (25-900C.') and excellent creep resistance to temperatures up to about 1,000C. In addition, the

molar ratio ZnO:Li O of 1:4 1:9 and the molar ratio which, on being melted together, will be converted to A1 O :(Li O +Zn+ nR,,.O,,) above0.95 must also be 'the desired oxide composition in the desired propormaintained. ZrO may be included as a secondary nutions. The batch ingredients were dry ballmilled tocleant in amounts up to about 2% by weight but its gether to aid in obtaininga homogeneous'melt and then presence can lead to premature phase separation and placed into platinum crucibles. The crucibles, imperfect growth of gahnite and/or the R,,,O,,-related equipped with a platinum stirring device, were placed crystal phase. Therefore, in general, TiO is employed in a gas-fired furnace and the batches melted at as the sole nucleating agent where the higher creep re- ,650C. for about 16 hours. Glass cane about sistance and dimensional stability are desired. Onerfoufth inch in diameter were hand drawn from [0 each melt and the remainder ofthe melt rolled between Table 1 records the compositions of several thermally steel rollers into plates about 4 inches X /2 inch X 68 crystallizable glasses, expressed on theoxide basis both inches in length. The glass was immediately transferred in terms of weight percent and in molar ratios, which to an annealer operating at about 650-700C. AS203 are illustrative of examples operable in the present inwas included in each of the batches to serve as a fining vention. The actualbatch ingredients therefor may be 15 agent although it will be recognized that other convenany materials, either oxides or other compounds, tional fining agents could be substituted therefor.

TABLE 1 (Weight Percent) 1 2 3 .4 5 6 7 8 SiO, 63.5% 64.3% 70.2% 70.2% 70.8% 70.6% 71.2% 71.4% v 1 0:, 22.1 21.2 17.3 17.4 17.5 I 16.9 17.0 16.3 L1 0 4.6 4.6 3.7 3.8 3.8 3.5 3.7 3.6 ZnO 1.9 1.9 1.5 1.6 1.6 2.1 1.6 1.5 TiO,- 4.2 r 4.8 4.1 4.3 3.5 4.2 3.8 4.4 AS 0 0.7 0.7 0.7 0.8 0.8 0.8 0.8 I 0.9 S 3.0 2.5 2.4 2.0 2.0 2.0 2.0 1.9

SiO, 63.6% 69.5% 70.1% 70.5% 64.2% 69.7% 71.1% 71.3% A1 0; 21.0 17.2 17.3 16.9 21.2 17.6 17.6 16.3 ,0 4.6- 3.8 3.8 3.7 4.6 3.6 3.8 3.6 ZnO 1.9 1.6 1.6 1.5 .1.9 1.5 1.6 1.5 T10 4.7 4.2 3.5 3.8 4.7 4.1 3.6 4.4 A5 0; 0.7 0.8 0.8 0.8 0.7 0.7 0.8 0.9 BaO- 3.6 3.0 3.0 2.9

SiO 71.5% 63.4% 68.9% 70.7% 70.7% 71.1% A1 0; 17.1 20.9 17.4 16.2 17.5 17.0 U 0 3.7 4.6 3.6 3.5 3.8 r 3.7 ZnO 1.6 1.9 1.5 1.5 1.6 1.6 TiO, 3.8 4.7 4.1 4.3 3.5 3.8 A5 0, 0.8 0.7 0.7 0.9 0.8 0.8 .0 1.4 La O 3.8 3 8 3 0 2 1 2 1 (Mole Ratio) SiO, 45.00 45.00 62.00 60.00 60.00 62.00 62.00 65.00 A1 0 9.25 8.75 9.00 8.75 8.75 8.75 8.75 8.75 Li O 6.50 6.50 6.50 6.50 6.50 6.10- 6.50 6.50 ZnO 1.00 1.00 1.00 1.00 1.00 1.40 1.00 1.00 TiO, 2.25 2.50 2.75 2.75 2.25 2.75 2.50 3.00 A5 0 0.15 0.15 0.20 0.20 0.20 0.20 0.20 0.25 SrO 1.25 1.00 1.25 1.00 1.00 1.00 1.00 1.00

SiO, 45.00 60.00 60.00 62.00 45.00 62.00 60.00 65.00 A1 0, 8.75 8.75 8.75 8.75 8.75 9.25 8.75 8.75 1.1,0 6.50 6.50 6.50 6.50 6.50 6.50 6.50 6.50 ZnO 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 TiQ, 2.50 2.75 2.25 2.50 2.50 2.75 2.25 3.00 A5 0, 0.15 0.20 0.20 0.20 0.15 0.20 0.20 0.25 BaO 1.00 1.00 1.00 1.00 Y,O 0.50 0.625 0.333 0.50

S10, 62.00 I 45.00 62.00 65.00 60.00 62.00 A|,o, 8.75 8.75. 9.25 8.75 8.75 8.75 Li,0 6.50 6.50 6.50 6.50 6.50 6.50 ZnO 1.00 1.00 1.00 1.00 1.00 1.00 T10, 2.50 2.50 2.75 3.00 2.25 2.50 As,0 0.20 0.15 0.20 0.25 0.20 0.20 Y, O, 0.333 La,( 0.50 0.625 0.50 0.333 0.333

Crystallization of the glass'body takes place more radpily at highertemperatures within the crystallization range than in the cooler extreme thereof. However, it is readily appreciated that as the crystallization phenomenon begins, the proportion of glassy matrix to crystals is very large such that the glass body will deform if the temperature is raised too rapidly approaching and exceeding the softening point of the glass. Hence, the rate at which the temperature is increased into the crystallization range must not be so rapid that the growth of crystals is insufficient to support the body against deformation. Where the glass body is of a geometry such as to permit some means of physical support, very rapid rates of heat treatment can be employed. In general, however, temperature increases not exceeding about 300C./hour have been found suitable for most applications.

Although normal practice contemplates cooling the parent glass body to about room temperature for visual examination of glass quality before crystallizing in situ, such as not necessary. Where desired, the parent glass article need only be cooled below the transformation range of the glass and thecrystallization heat treatment begun immediately thereafter. The transformation range is that temperature at'which a liquid have been deemed to have been transformed into an amorphous solid, this temperature usually being-considered as lying between the strain and annealing points of a'glass.

' Commonly. the crystallization procedure foll'owe'd is similar to the schedules-set out in Table ll below. That is, specific dwell periods are employed at'temperatures within the nucleating and crystallization ranges. Never theless, such specificity is frequently merely a matter of convenience .since the glass-can be very satisfactorily crystallized by simply moving it withinjthose two ranges with no finite dwell period of any duration..ln sum, the heat treatment schedule only demands: 1) that the glass remain within thenucleation range for a sufficient length of time to insure a substantial development of nuclei such that the crystals grown thereon will be fine grained; and (2) that the'nucleated glass remain within the crystallization] range for a sufficient length of time to achieve the desired highly crystallinejproduct.

' Inasmuch as the coefficient of thermal expansion of the crystallized body is so low as to approach zero, little care need be exercised in cooling the article from the crystallization range. Hence, the crystallized articles normally can be removed directly from the heat treating chamber and permitted to cool inthe ambient atmosphere. However, in the examples recorded in Table ll, it was convenient to simply cutoff the current to the electrically-heated furnace and allow-the furnace to t cool to room temperaturewith the crystallized articles inside. That practice. termedco'oling at furnace rate, has been estimated at about 3-5C./minute.

Table ll reports the heat treatment schedulesto which the examples 'were subjected, each schedule being given a letter designation. Table III records the heat treatment schedule, in terms of letter designation,

to which'each example was subjected along with the.

crystal phasescontained therein as identified through X-ray diffraction analysis and the coefficient of thermal expansion (25-900C.) as measured in the conventional manner utilizing a ditferential dilatometer. Also,

a measure of the creep resistance of the crystallized bodies at l,000C. was determined in terms of apparent viscosity. utilizing the beam bending method described by H. E. Hagy in Experimental Evaluation of Beam- Bending Method of Determining Glass Viscosities in the Range l0 to 10' Poises, Journalof the American Ceramic Society, 46, No. 2, pp.9397, February, 1963.

A determination of the apparent viscosity demonstrated by the crystallized body after being held for 1 hour at 1,000C. provides a comparatively simple and rapid means for estimating the relative high temperature stability to be expected from a material after extended periods of operation." inasmuch as'the beam bending procedure contemplates a measurement of I sample deformation under an applied load, commonly l,500-2,000 psi, it cannot provide a directreading as to dimensional changes which an article will undergo at elevated temperatures with no load applied thereto. Thattype of measurement is secured through the length comparator test described by Wilmer Souder and Peter Hidnet, Measurement of the Thermal Expension of Fused Silica,-Scientific Papers of the Bureau of Standards, 21, pp. 1-23, Sept. 21, I965. Nevertheless, experience with the two methods has demonstrated that materials exhibiting high apparent viscosity values, as determined by the beam bending technique, .will also normally manifest superior creep resistance and long-term dimensional stability at high temperatures when measured according to the length comparatortest. I The Y- Phase" reported 'in Examples l3 l7is'characterized by strong X-ray diffraction lines at d-spacings of about 2,.93A, 2.77Ay and l. 74A. The Y Ti Q phase,

which commonly appears in similar compositions nucleated with TiO but which do not contain ZnO (Ser. No. "82,844, supra), is present in very minor amounts, if at all, after the crystallizationheat treatment. it has been discovered that subjecting the crystallized bodies of the instant invention to temperatures within the l,000l,lO0?C range for very extended periods of time, e.g. greater than about 24 hours, causes the Y Phase to slowly revert to Y Ti O A second feature of such prolonged heat treatments is the frequent increase in the amount of rutile observed. These observations have suggested that the Y Phasemay be a metastable form of Y Ti O (Y O -2TiO or acompound of the form Y QrnTiO -Wherein N 2.

characteristic of La- Ti O occur in' each composition with several also showing lines which could sult of the presence of vLa O V TABLE II' be the re Letter Designation Heat Treatment Schedule A Room temperature to 800C. at. 200"C./hour Hnld at 800C. for 2 hours Raise temperature to 1l50C. at 200C./hour Hold at l C. for 4 hours Cool to room temperature at furnace rate.

B Room temperature to 800C. at 200C.lhour Hold at 800C. for 4 hours Raise temperature to'l ISO C. at 200C.lhour Hold at 1 150C. for 4 hours Cool to room temperature at furnace rate.

Room temperature to 850C. at 300Cz/hour Hold at 850C. for 2 hours TABLE ll- Continued Table IV records the composition of several thermally erystallizable glasses, expressed on the oxide Letter basis both in terms of weight percent'and in molar ra- Dwgmum" schcdulc ttos, which are close to but outside the scope of the S W5 a! 5 present invention, primarily because the ZnO:Li O rag mom g' gfg furnace mm. ttos are not within theranges spelled out therefor. The glass articles were fabricated m like manner to those reported 1n Table I. Table IV also recites the crystalli- D Roam pg to at C./hour zation heat treatment applied to each and the crystal sg f y sggsi fi i m loose/hour l phases present 1n each as identified through X-ray dif- Hold at 11501. for 4 hours fraction analyses. Cool to 850C. at 200C./ho ur Held at 850C. for 2 hours TABLE IV Raise temperature to I050C. at 2()0C./hour Hold at l'050C. for H) hours Cool to room temperature at furnace rate. (Weight Percent) 23 24 25 26 27 r a .E Room temperature to 825C.- at 300C./hour Held at 825C. for l hour Raise temperature to-l()C. at 200C.lhour U20 '2 Hold at l200C. for 2 hours Zno O9 Cool 10 l050C. at 200C./hour 2O l0 Hold at I050C. for 2 hours 4 s 2 .l Cool to room temperature at l'urndu. rate. i 4.5 4'2 3'8 3 3.8 As o 0.9 0.8 0.8- 0.8 0.8 (Mole Ratio) F Room temperature to 825C. at 300C./hnur 23 24 26 27 Hold at 825C. for l hour 25 S102 6Z0 2- 2-0 2-0 Raise temperature to l 120C. at 200C./hour lz n .75 Hold at 1 120C. for 2 hours L 2 -50 5-50 550 Cool to I050C. at.200C./hour ZnO 0.50 0.60 2.00 2.00 2.00 Cool to room temperatureat furnace rate. -0 V103 0.33

TABLE III Expansion Coeffieien Apparent Viscosity Example No. Heat Treatment Crystal Phases (MO 1C.) (Poises) I A Beta-spodumene s.s., Gahnite. Rutile,

SrO'Al O '2SiO= t 2 A do. 9.5 3.6 X [0 3 A do. I 4 A Beta-spodumene s.s., Gahnite. Anatase, 3.7 2.0 X l0" Sr0-Al,O,-2SiO,' 4 E do. 3.7 8.6 X I0 5 'C Beta-spodumene s.s.. Gahnite, Rutile,

. SrO-QOyZSiO, 5 D do. 3.8 6 A do. 7 C do. 8 A do. 8 C do. 9 A Beta-spodumene s.s. Gahnite, Anatase. 8.4 4.7 X 10 Ba0-Al,O -2Si0 9 C do. 10 B do. 7.4. 8.0 X l0" l0 F do. 5.0 8.6 X l0 1 l C Beta-spodumene s.s., Gahnite. Rutile,

BaOAl,O -2SiO, 11 D do. 42 2.6 X l0 l2 C do. 13 A Beta-spodumene s.s., Gahnite, Rutile. 8.3 2.8 X IO' Y Phase" 13 C do. 14 A -Beta-spodumene s.s.. Gahnite, Rutile,

Y Phase", YTlz0 15 D Beta-spodumene s.s., Gahnite, Rutile, 4.0 3.5 X l0 Y Phase" 16 A do. 17 E do. 3.4 9.5.x 10" l8 A Beta-spodumene 5.5., Gahnite, Rutile, l0.l 4.4 X l0 Lafli o, v 18 C Beta-spodumene s.s., ahnite, Rutile,

La,Ti 0,, La,O 19 A 'Bcta-spodumene s.s.. Gahnitc. Rutile.

La,'l'i 0 -20 A do.

20 C Beta-spodumene s.s.. Gahnite. Rutile,

' l.a,Tl,O Lagos 2| D do. 5.5 4 6 X l0" 22 E do. 5.4 l 3 X l0" TABLE lVContinued (Weight Percent) La,Ti,0 C ristobalite Examples 23 and 24 are representative of compositions wherein the ZnO content is too low to develop the required gahnite phase. In contrast, Examples 25-27 are illustrative'of compositions wherein the ZnO:Li O ratio is greater than. 1:4. viz., 2:5.5..The silica polymorph cristobalite is fonned which exhibits a high coefficient of thermal expansion that can lead to cracking of the body due to unequal stresses developing therein and may also contribute to dimensional instability.

Table V provides a comparison of the melting and forming behavior of Examples 6 and 1,0 with glass compositions similar to those of the instant invention but free from zinc oxide. Thus, the liquidu's temperature is recorded for each along with the viscosity, in. poises, of

the'melt at that temperature. Also.'a reading is given of vthe temperature at which the melt exhibits a viscosity of 10", 10. and 10 poises. As can be clearly seen, theaddition of ZnO lowers the liquidus temperature very markedly but without softening the glass to any substantial extent.

TABLE V (Weight Percent) SiO 71.6% 70.2% 70.9?! 69.5% A00, 17.1 I 17.4 17.0 a 17.2 L1 0 4.3 3.8 4.3 3.8 zero 1:6 s SrO 2.0 2.0 BaO 2.9 3.0 TiO v 4.2. 4.2 4.2- 4.2 As o 0.8 i 0.8 0.8 0.8 (Mole Ratio) 28 '4 29 t0 SiO,. 62.00 60.00 62.00 60.00 A00 8.75 8.75 8.75 8.75 Li,0 7;50 6.50 7.50 6.50 ZnO, 1.00 1.00 SrO 1.00 1.00 BaO 1.00 1.00 TiO, 2:75 2.75 i 2.75 2.75 As,0, 0.20 0.20 0.20 0.20 .Liquidus "C. 1313 I 1285 1307 1265. Liquidus Viscosity 23.000 33.000 7 34.000 47.000 1 1557 1560 15,75 1550 10 1373 1365 1393 1368 10 1223 1215 1237 1217 Tables l-V amply illustrate the composition and process parameters which must be observed to yield glasses having improved forming characteristics and coefficients of thermal expansion coupled with excellent creep resistance at temperatures upto 1.000C.

final glass-ceramic bodies exhibiting the desired low and higher. Hence, as is clearly pointed out in Table V. the substitution of ZnO for Leo to. the base Li- O-Al O -SiO, glasses causes the lowering of the liquidus temperature of the glass without concurrently causing a substantial decrease in the viscosity of the melt at any specific temperature. Such a phenomenon greatly improves the forming behavior of the glass. The crystal content of the glass-ceramic body is in excess of by volume and, frequently, exceeds by volume, being a function of the heat treatment schedule employed and teh extent to which the components of the batch are adaptable to the formation of crystal phases. The individual crystals are relatively uniformly fine-grained, essentially all being smaller than five microns in diameter and the majority being less than about two microns.

Heat treatment schedules E and F offer a very desirable combination of low coefficient of thermal expansion with good creep resistance after relatively short periods of crystallization. These schedules appear to permit the very adequate development of both the pri-, mary and secondary crystal phases with a consequent minimum of residual glass in the. crystallized body. The judicious selection of heat treating schedulewill allow the tailoring of a final product to exhibit outstanding creep resistance.- Thus, heat treatment schedule A. when applied to Example 18, imparted an apparent visco'sity thereto of up to an order of magnitude greater than normally measured on the La o -containing glasspansion (25900C.) below about 10 X l0' /C. and

excellent creep resistance and dimensional stability up toabout l.000 C. wherein beta-spodumene solid solution constitutes the principal crystal phase and gahnite constitutes asecondary crystal phase which comprises:

a. melting a batch for a glass composition consisting essentially. by weight on the oxide basis. of about 62-73% SiO 15-23% A1203; 3'5% Li O, 1-3% ZnO, 3-6%'TiO and l-5% R O wl1 erein-R,,,O consists of 'SrO, B'aO, Y O La O and mixtures thereof, and wherein the molar ratio ZnOELi O varies between about 1:4 1:9 and the molar ratio A1- O :(Li O ZnO-+ nR,,,O,,) is greater than 0.95;

b; simultaneously cooling the melt at least below the transformation range thereof and shaping a glass article of a desired configuration therefrom;

c. heating said glajss article to a temperature between about 750900C. for a period of time sufficient to cause substantial'nucleation of said article; g d. heating said article to a temperature between about 900+l ,250C. for a. periodof timesufficient to cause the article to crystallize in situ; and then in situ' ranges between about l-l2hours. 

2. A method according to claim 1 wherein said period of time sufficient to cause substantial nucleation ranges between about 1/4-6 hours.
 3. A method according to claim 1 wherein said period of time sufficient to cause the article to crystallize in situ ranges between about 1-12 hours. 